1. Field of the Invention
The present invention relates to a magnetic field detection sensor.
2. Description of the Related Art
Having been conventionally disclosed is a magnetic field detection sensor including a magneto-impedance (MI) element that makes use of the magneto-impedance effect of an amorphous wire. Downsizing is easier with an MI element, compared with a flux gate sensor, and sensors using an MI element exhibit detection sensitivity equivalent to that of flux gate sensors.
An example of such magnetic field detection sensors using the MI element will now be explained. An oscillator circuit applies an alternating-current (AC) bias current to a coil wound around an MI element. An AC bias magnetic field is generated thereby in the MI element, and an amplitude-modulated (AM) waveform whose peaks have a height difference alternately is acquired as an output. A detector circuit is configured to detect the output, and removes a direct-current (DC) component, and the resultant waveform is input to a comparator. The magnetic field detection sensor then acquires an output signal with a digital waveform having been pulse-modulated based on the height difference. Such a magnetic field detection sensor acquires the strength of the external magnetic field, not based on the absolute value of the amplitude of the output from the oscillator circuit, but based on the amount of change in the amplitude. Therefore, such a magnetic field detection sensor is less affected by a variation in the source voltage, the temperature characteristics of the element, and the like, so that not only precise adjustment of the oscillator circuit is rendered unnecessary, but also a magnetic field detection sensor robust against noise can be provided (see Japanese Patent Application Laid-open No. 9-127218).
Also disclosed is a magnetic field detection sensor including an oscillator circuit that applies a high-frequency sine wave current to both ends of a magnetic core, a buffer circuit that is disposed between the oscillator circuit and the magnetic core of a thin-film magneto-impedance element, and adjusts mismatch between an output impedance of the oscillator circuit and an input impedance of the thin-film magneto-impedance element, a detector circuit for detecting an amount of magnetic change in the external magnetic field based on the amount of change in the high-frequency current which changes according to the external magnetic field applied to the magneto-impedance element, and a hysteresis cancelling circuit that cancels the hysteresis in the magneto-impedance element. Because such a magnetic field detection sensor is provided with a buffer circuit, for example, the output resultant of the high-frequency current applied by the oscillator circuit can be supplied without any loss (see Japanese Patent Application Laid-open No. 2000-180521). In the magnetism detection sensor, because the inclination representing the amount of change, which is dependent on the external magnetic field applied to the magneto-impedance element, becomes steepest at 400 A/m, as illustrated in FIG. 5 in Japanese Patent Application Laid-open No. 2000-180521, a constant current that moves the operating point to the point where the amount of change becomes steepest is applied to the bias coil.
The magnetic field detection sensors disclosed in Japanese Patent Application Laid-open No. 9-127218 and Japanese Patent Application Laid-open No. 2000-180521 are, however, incapable of making highly sensitive measurement unless applied is an AC bias at a level at which the inclination of the impedance change becomes steep (up to ±400 A/m in FIG. 5 disclosed in Japanese Patent Application Laid-open No. 2000-180521), because the impedance that is dependent on the magnetic field of the MI element changes in an M-shape. Therefore, the current consumption is increased. Similarly, an approach for applying a DC bias at a level at which the inclination of the impedance change becomes steep requires an increased amount of current consumption.
Furthermore, because a hysteresis is increased in the M-shaped characteristics, the detection precision is deteriorated by the amount of the hysteresis. Moreover, because the magnetic field can only be measured highly sensitively within the range where a steep inclination is ensured, only a limited detection range has been available.